1. Field of the Invention
The present invention relates to a method and apparatus for adaptive sub-band allocation of spectral coefficients, and more particularly, to a method and apparatus for adaptive sub-band allocation of spectral coefficients, in which the sizes of sub-bands are determined according to the distribution of spectral coefficients transformed from an input speech/audio signal to perform quantization in units of sub-bands.
2. Description of the Related Art
An analog speech signal is transformed to a PCM (Pulse Code Modulation) signal through sampling and quantization. Such signal transformation requires a large capacity for processing, and is accompanied by many difficulties in storage, transmission, and reproduction due to large capacity.
Therefore, a lot of speech/audio codecs have been developed to compress and restore the PCM signal.
Narrowband codecs for decoding speech having a bandwidth of 300 Hz˜3,400 Hz achieve a high compression rate based on LPC (Linear Prediction Coefficient) technique in which a speech generation process is modeled.
In addition, speech/audio codecs of a broad bandwidth (50˜7,000 Hz), a superbroad bandwidth (50˜1,400 Hz), and a full bandwidth (20˜22,000 Hz) use a method of transforming an input signal from a time domain into a frequency domain and quantizing it.
Representative frequency domain transformation methods include DCT (Discrete Cosine Transform), DST (Discrete Sine Transform), DTF (Discrete Fourier Transform), MDCT (Modified Discrete Cosine Transform), and so forth.
MPEG audio codecs employ a method of allocating bits and quantizing spectral coefficients by using a psychoacoustic model, and codecs such as G.729.1 and G.7111.1 employ a method of dividing spectral coefficients into sub-bands having a fixed size and scalar-quantizing the gain of the spectral coefficients in the sub-bands and vector-quantizing the shape thereof.
However, the aforementioned conventional method of allocating spectral coefficients using fixed sub-bands is problematic in that, if the spectrum distribution of sub-bands is high at a specific coefficient, there is a limitation to achieve accurate rendering by vector quantization, thereby causing sound quality degradation.
Moreover, even when the spectrum distribution is uniform on the whole, if a fixed sub-band is used, the distribution of bits is inefficient, and excessive computation compared to signals is carried out. Thus, improvements on these problems are demanded.